Rotating machinery will typically introduce both acoustic and vibration energy into any fluids or structures surrounding the machinery. The acoustic and vibration energy can be caused by both random and deterministic processes related to the operation of the machinery. Random processes result in noise or vibration that is spread over a wide band of frequencies. Deterministic processes, on the other hand, often generate energy that is confined to a family of distinct frequencies radiated as "pure" tones. Typically, for a machine having a rotating shaft, the tones are radiated at frequencies that are integer multiples of the frequency at which the shaft rotates. As a result, rotating machines radiate acoustic and vibration spectra that contain both broadband and tonal components.
Those skilled in the art are aware that systems which radiate strong levels of tonal noise are annoying to humans. In fact, noise control engineers are aware that tonal noise can be more annoying to humans than other forms of acoustic noise. As a result, the noise control engineers often focus on the reduction of tonal noise generated by such rotating machinery.
In reducing the tonal noise generated by multiple rotating machines operating in the same environment, of particular concern is the tonal noise generated when two or more machines spin at nearly the same rate. That is, those interested in reducing tonal noise radiated from a plurality of rotating machines within close proximity to each other are particularly concerned with the tonal noise generated when each machine radiates tones at a slightly different frequency than the other machines. In such an environment, the frequencies of the tones radiated from each machine will vary slowly with respect to each other. As a result, the tones will constructively and destructively interact to create tones that "beat". That is, the tones radiated from each machine will interact with each other such that a noise having a given envelope variation ("beating effect") will be perceptible to the human ear. Such a beating effect can be heard in propeller driven aircraft, or in rooms where multiple fans are running. As most are aware, the beating noise can be considered to be particularly annoying to the human ear.
Moreover, since a structure can be characterized by the natural resonant frequencies at which it most easily vibrates, those skilled in the art are also concerned with vibration energy radiated by such rotating machinery. That is, if the vibrational frequency(s) generated by a rotating machine match the natural resonant frequency(s) of a given structure near the machinery, the structure can be forced into strong vibration. The result can be structural fatigue and/or additional acoustic noise generation. As a result, those skilled in the art are continuously concerned with minimizing the effect of such acoustic and vibration energy radiated by rotating machinery.